In photogrammetric aerotriangulation, plotting is reduced to the measurement of coordinates in aerial photographs by analytical measuring methods. Conventional plotting apparatus available for this purpose require completely satisfactory surveying or measurement of image points of several images with respect to one another. This surveying is much more accurate if characteristic points are already precisely marked on the images by photographing signalled or indicated ground features or terrain points, or by applying identical marking symbols to corresponding images.
Signalling of terrain points is expensive due to the required work in the field. If signalled terrain points are not provided, the use of non-signalled natural features subjects interpretation to the risk of identification errors, and sometimes renders interpretation impossible, e.g., in terrain with few details. Thus, marking processes and apparatus have been provided for precisely positioning artificial marking symbols on corresponding image points of several images. Methods and corresponding apparatus are already known to artificially mark the surface of the photographic material for photogrammetric triangulation. Mechanical, electrical, electro-optical and thermal methods exist for this purpose, but suffer from serious disadvantages.
Cutting a point in a photographic emulsion using a rotary, specially shaped steel, diamond or sapphire stylus creates non-clearly defined, non-circular holes and emulsion particles (cuttings). The particles close the holes again or deform the holes in some other way causing serious measuring errors.
Impressing the emulsion by briefly pressing a very small diameter steel ball or a sapphire needle in the emulsion leads to rounded impressions, and consequently, to poorly defined hole boundaries. The impression formed holes can also be made indistinct through residues. Finally, the elasticity of the emulsion causes the holes made by this impression method to have only a very short life. Additionally, it is not possible to make holes with a sufficiently small diameter.
The marking of the emulsion by an electric discharge has proved to be too inaccurate and has not been adopted in practice. This is because the path of the spark produced and its striking point on the image surface cannot be sufficiently controlled.
Holes burnt into the emulsion by optical beams, particularly laser pulses, initially require a significant technical expenditure. In addition, such methods are dangerous to the operator's eyes, and special protective measures are required for observing the images during marking. Moreover, the shape of the holes obtained is not of an adequate quality.
In thermal methods, an electrically heated stylus is briefly pressed into the emulsion. At the impression point, the emulsion is displaced or evaporated forming a clearly defined, circular, black-looking bead providing stable and regular marking symbols with a high contrast at bright image points. The stylus can be heated in a number of ways, e.g., by a heating coil, inductively, through the discharge of a capacitor through a stylus contructed as an electrical resistor, and by pressing a preheated crystal into the emulsion. However, such thermal methods are disadvantageous since the stylus heating required relatively high temperatures of up to 300.degree. C., the irradiated heat adversely influences the accuracy of the apparatus, and the preheating of the stylus leads to a relatively long marking cycle reducing productivity.